/Engine/Source/Runtime/Core/Public/Misc/Paths.h
Returns platform and restricted extensions that are present and valid (for platforms, it uses FDataDrivePlatformInfo to determine valid platforms, it doesn't just use what's present)
BaseDir and usable extension directories under BaseDir (either Engine or Project)
static [TArray](API\Runtime\Core\Containers\TArray)< [FString](API\Runtime\Core\Containers\FString) > GetExtensionDirs ( const [FString](API\Runtime\Core\Containers\FString) & BaseDir, const [FString](API\Runtime\Core\Containers\FString) & SubDir )